1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device including a plurality of gate insulating films with different film thicknesses.
2. Description of the Related Art
Semiconductor devices of the above-noted type include non-volatile semiconductor devices such as a flash memory. The flash memory comprises a memory cell region and a peripheral circuit region both necessitating different gate breakdown voltages. Accordingly, gate insulating films having different film thicknesses corresponding to the different gate breakdown voltages respectively. In one of methods of fabricating semiconductor devices, a film structure of gate electrodes is formed first and subsequently, a structure of gate electrodes is formed together with formation of an element isolation region. When the gate insulating films have different film thicknesses in the aforesaid fabrication method, the following defect occurs: when a trench serving as shallow trench isolation (STI) is first formed and subsequently, an insulating film buried in the trench is planarized by a chemical mechanical polishing (CMP) process, the difference in the film thicknesses of gate insulating films results in differences in the level of a silicon nitride film serving as a stopper film in the CMP process. Accordingly, when polishing reaches part of the silicon nitride film at a lower side in the CMP process, part of the silicon nitride film is cut away to an abnormal extent corresponding to the level difference. As a result, an amount of process margin becomes smaller as the level difference in the silicon nitride film is increased. Furthermore, the level difference easily tends to result in discrepancies such as dishing in the CMP process.
To overcome the aforesaid discrepancies, U.S. Pat. No. 6,642,105B2 discloses a semiconductor device in which a thick oxide film is formed by a method of selective oxidation such as local oxidation of silicon (LOCOS) on a part where a gate oxide film with a larger film thickness is formed. The thick oxide film is etched so that a necessary film thickness is obtained, whereupon the difference due to the film thickness difference is reduced.
Thus, in the above-noted U.S. Patent, in order that the difference in the film thickness of the gate oxide films may be resolved, a thick oxide film is formed by the selective oxidation on a part of the silicon substrate on which a thick gate oxide film is to be formed in a region where a thick gate oxide film is to be formed. The thick oxide film is etched so that a suitable film thickness is obtained, whereby a gate oxide film with a desired film thickness is formed. However, the method disclosed in the above-noted U.S. patent has the following disadvantage: an oxide film formed by the selective oxidation, namely, LOCOS, results in formation of a distorted region called “bird beak” formed in a peripheral edge of the oxide film. The distorted region cannot be used as contributing to the characteristic of the device. Consequently, the distorted region needs to be taken into consideration in the design of the semiconductor device, resulting in constraints to reduction I an area of the semiconductor device.